The death toll from the collapse of the roof of a five-storey cement factory in Bangladesh has risen to at least seven, a government official said.

Rescue work is under way but there is no estimate for the number of people still trapped, said an official in Mongla in the Bagerhat district where the accident happened.

About 150 people were working when the collapse happened yesterday at the seaside factory 85 miles (135 kilometres) southwest of Dhaka.

The cause of the collapse is under investigation.

Survivors said 50 to 60 people were working on the roof while others were on the ground floor when the collapse occurred. It was not clear how many were directly affected by the collapse.

The factory is reportedly owned by an army welfare organisation.

Building structural safety is a problem in Bangladesh, with engineering surveys showing that many factories violate building codes by using substandard materials or adding extra floors that the building foundations cannot support.

In April 2013, a building housing numerous garment factories collapsed in Dhaka, killing more than 1,100 people.